SH53A-0301 1340h
Re-analysis of the long-term changes of the NIMBUS-7 radiometer and behaviour of total solar irradiance during solar cycle 21
Only one radiometer - called Hickey-Frieden (HF) - is within the ERB package on NIMBUS-7 for the measurement of total solar irradiance (TSI) and thus changes due to exposure to solar radiation cannot be directly determined by comparison with a less exposed radiometer on the same spacecraft. The geometry and optical property of the cavity of HF is, however, very similar to the PMO6-type radiometers, it is essentially a copy of it with increased size. For the PMO6V on VIRGO/SOHO two main effects have been identified, a rapid early increase and a slow decrease, normally termed degradation, which can be modelled with a hyperbolic function taking the actual dose into account (Fr\"{o}hlich, 2003). The corrections used by Fr\"{o}hlich and Lean (1998) for the composite were based on early results from VIRGO and used simple exponential functions. With the recent results from VIRGO a re-analysis of the long-term behaviour of HF is necessary. The results are not only important for solar radiometry from space, but also to improve the reliability of TSI before the start of ACRIM-I in early 1980. The latter will allow to better quantify the behaviour of solar cycle 21 and to compare it with the two recent ones which differ in several aspects. C. Fr\"{o}hlich. Long-term behaviour of space radiometers. Metrologia, 40:60--65, 2003. C. Fr\"{o}hlich and J. Lean. The sun's total irradiance: Cycles and trends in the past two decades and associated climate change uncertainties. Geophys. Res. Lett., 25:4377--4380, 1998.
SH53A-0302 1340h
TIM Degradation Rates and Possibilities of Measuring Secular TSI Changes
We present the on-orbit degradation measured on the Total Irradiance Monitor (TIM), a total solar irradiance (TSI) monitoring instrument on NASA's SOlar Radiation and Climate Experiment (SORCE). The monitored degradation to date is extremely small (50 ppm/yr) with an uncertainty of 10 ppm/yr. We discuss the possibilities of accurately measuring secular changes in TSI using instruments with comparable degradation rates.
http://lasp.colorado.edu/sorce/
SH53A-0303 1340h
Implications of Ground Based Photometric Images for Long Term Solar Irradiance Variations
The San Fernando Observatory (SFO) has produced photometric full disk solar images at 5 arc second resolution since 1986, and 2.5 arc second resolution since 1989. We have previously shown that the best quantities for solar irradiance modeling are the {\em photometric sums} $\Sigma$, defined as the summed residual intensity on these photometric images. In particular, a linear regression of total solar irradiance $S$ to the time series of $\Sigma_r$ and $\Sigma_K$, the photometric sums in broadband red and Ca~II~K filters, respectively, does an excellent job of reproducing $S$ during cycle 22 (Preminger, Walton, and Chapman 2002, {\it JGR} {\bf 107}, Issue A11, SSH 6-1). We have also shown (Walton, Preminger and Chapman 2003, {\it Solar Phys.} {\bf 213}, 301) that variations in the chromospheric network appear to account for no more than 25% of the change in $S$ over the solar cycle. In this talk, we extend these results to cycle 23 and discuss their implication for long term changes in $S$. In particular, if $\Sigma_r = \igma_K = 0$ can be taken as representing the complete absence of solar activity, then one would conclude that the minimum level of $S$ is not much below those currently observed at solar minimum; quantitatively, about 0.3 W~m$^2$ below that level.
SH53A-0304 1340h
Inferring Total and Spectral Solar Irradiance From Sunspot Areas Only
We show that daily sunspot area can be used in simple models to reconstruct daily variations in total or spectral solar irradiance. The models assume that all solar irradiance fluctuations can be traced back to the emergence of sunspots on the solar disk. Cotemporal data for irradiance and sunspot area are analysed to extract a detailed impulse response function that describes the time evolution of the irradiance in response to a sunspot that is a delta function. The analysis is carried out using data for total solar irradiance (the Fr\"ohlich-Lean TSI composite) and spectral solar data (the Mg~ii core-wing ratio and the 10.7cm solar radio flux F10.7). The details of the impulse response functions are different in each case, but overall they clearly show the evolution of a dark sunspot into a well-defined bright region which then spreads out and decays over a period of about 500 days. The impulse response functions can be used to produce a daily record of spectral or total solar irradiance from the Greenwich daily sunspot area database, which extends from the late 1800's to the present. The reconstructions of TSI, Mg~ii core-wing ratio and F10.7 show little long-term trend in their levels at solar minimum.
SH53A-0305 1340h
Solar UV Spectral Radiance from SKYLAB
As part of an ongoing NASA-LWS sponsored research program, solar UV spectral radiance has been determined for the quiet sun at a variety of center-to-limb positions observed by the SO82B UV spectrograph on SKYLAB. This has been done using a high quality flatbed scanner to digitize the SKYLAB film data. The goal of the present program is to develop a spectral irradiance model that utilizes the full resolution of the SKYLAB spectra (~ 0.010 nm at 300 nm). The model requires detailed knowledge of the center-to-limb variation (CLV) at full resolution over the 200 - 400nm wavelength range. This information is also required to determine the absolute intensity calibration which uses full-disk irradiance measurements from UARS (SOLSTICE and SUSIM) in addition to the CLV. This calibration is then used to derive the disk center radiance. In addition to the quiet sun, spectra of sunspots and active regions are also being digitized and an attempt is underway to derive the CLV for these surface morphologies as well. Further, spectra at shorter wavelength (~120 - 200nm) are being examined in order to extend the wavelength coverage of these radiance and CLV results. In this presentation we will present the details of the analysis methods and results for the quiet sun. Ongoing work on sunspots and active regions will be presented as will results involving work on shorter wavelength spectra.